<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>

  <style>
    .box {
      width: 500px;
      height: 500px;
      margin: 0 auto;
      background-color: green;
      /*开启flex布局*/
      display: flex;
      align-items: center;
    }

    .item {
      width: 100px;
      height: 100px;
      color: #fff;
      text-align: center;
      line-height: 100px;
    }

    .item1 {
      background-color: #f00;
    }

    .item2 {
      background-color: #0f0;
      height: 150px;
    }

    .item3 {
      background-color: #00f;
      /*flex item可以通过align-self覆盖flex container设置的align-items*/
      /*auto: 默认值, 遵从flex container的align-items设置*/
      /*strech、flex-start、flex-end、center、baseline效果跟align-items一致*/
      align-self: flex-end;
    }
  </style>
</head>
<body>
<div class="box">
  <div class="item item1">item1</div>
  <div class="item item2">item2</div>
  <div class="item item3">item3</div>
</div>
</body>
</html>